Biomass pretreatment for conversion to sugars remains one of the most challenging obstacles to the design of an economically viable bi-refinery for biofuel production. Several pretreatments and biomass fractionation technologies exist or are under consideration. A promising new candidate technology for biomass fractionation is ionic liquid (IL) pretreatment. In the IL pretreatment process, biomass is dissolved in an IL and carbohydrates, such as hemicellulose and cellulose, are precipitated on the addition of an anti-solvent, such as water. The regenerated cellulose possesses an amorphous structure that is especially amendable to enzymatic saccharification and may contain little or no inhibitory components for subsequent processing to a biofuel.
The most common approach to recover the ionic liquid is to simply distill off solvents (including water) that dilute ionic liquids or are formed as byproducts of reactions carried out in ionic liquids. While this is a simple and effective approach, it is also very energy intensive, especially when low concentrations of ionic liquid are encountered, or when the processing scale becomes large. Currently, there is nothing in the literature indicating any IL recovery approach involving solvents other than removal by distillation/evaporation except in the few cases where reactants or products are miscible in a second, immiscible liquid added for the purpose of reactant, product, or IL recovery.
The use of hydrocarbon solvents in ionic-liquid processing of cellulose is generally not emphasized in the literature because they are not as ‘green’ and approach as could be desired. All literature references I am familiar with describe ionic liquid recovery in terms of distilling off the precipitating solvent. No mention anywhere is made of the difficulty of recycling ionic liquid from the more dilute solutions resulting from ‘wash’ steps or other procedures designed to recover the relatively expensive ionic liquid from the inexpensive product stream.
Currently, different precipitating agents are used, but all involve the formation of a gel phase at low precipitating solvent concentrations and high cellulose or biomass solute concentrations. This is usually compensated for by increasing the amount of precipitating solvent used, but there are process costs associated with this additional solvent, and these can be minimized by working in the narrow phase region that we have discovered. We believe that the solvent mixture that we have found results in the minimum amount of precipitating solvent and a maximum quantity of dissolved cellulose or biomass to be used in a process, and is clearly superior to use of a single solvent or mixture outside of the specific composition range described below.
There is no generally known method of extracting lignin and sugars from ionic liquids used in ionic liquid biomass pretreatment. For some other processes, there have been ionic-liquid immiscible solvents found where the partitioning coefficients of the desired solutes are appropriate for liquid-liquid extraction. In some other chemical processes a product may be removed thermally (as a vapor) or can undergo a chemical reaction to form a solid product. However, none of these approaches is applicable to ILs that are useful solvents for biomass.
The present invention overcomes these issues.